An organic electroluminescent device of the simplest structure is generally the one constituted of a light-emitting layer sandwiched between a pair of counter electrodes. Upon application of an electric voltage between the electrodes of an organic EL device, electrons injected from the cathode and holes injected from the anode recombine in the light-emitting layer and the energy level after recombination returns from the conduction band to the valence band with emission of light; it is the phenomenon of this emission of light that is utilized by an organic EL device.
Organic EL devices and organic EL materials used therefore are known in a large number of documents. The use of phosphorescence instead of fluorescence has been investigated in an attempt to enhance the luminous efficiency of an organic EL device. Namely, a device utilizing emission of light from the triplet excited state is expected to perform at a higher efficiency, approximately three times the efficiency of a conventional device utilizing fluorescence (singlet). Numerous developmental works on phosphorescent dopants have been under way to achieve this end.
The following patent documents are known in relation to this invention.                Patent document 1: JP2003-515897 A        Patent document 2: JP2001-313178 A        Patent document 3: JP2002-305083 A        Patent document 4: JP2002-352957 A        
The patent document 1 describes complexes represented by L2MX as phosphorescent dopants for use in organic EL devices. In a preferable example of L2MX cited in the document, L is a substituted benzoxazole, M is iridium, and X is a ligand functioning as a trap for holes. The host material proposed for use in the light-emitting layer is a carbazole compound and 4,4′-N,N-dicarbazoylbiphenyl (CBP) is mentioned to be particularly preferable.
The patent document 2 discloses an organic EL device comprising a light-emitting layer in which 0.5 to 8 wt % of a phosphorescent iridium complex is incorporated. Here, tris(2-phenylpyridine)iridium or Ir(ppy)3 is cited as a preferable iridium complex and CBP as a preferable host material for the light-emitting layer.
A host material containing no phosphorescent dopant is useful as a material for a fluorescent EL device. However, a phosphorescent EL device has an advantage of enhanced luminous efficiency as mentioned above and, in the fabrication of a phosphorescent EL device, the compatibility of a host material with a phosphorescent dopant becomes an important factor. The aforementioned CBP has a property of facilitating the flow of holes and obstructing the flow of electrons and the problem with the use of CBP is that the balance of electron and hole injection is destroyed and excessive holes flow out to the electron transporting-layer side and, as a result, the luminous efficiency from Ir(ppy)3 drops.
The patent document 3 discloses an organic EL device which utilizes phosphorescence and comprises in its light-emitting layer an organic metal complex (1) containing a metal of Groups 7 to 11 in the periodic table and an organic metal complex (2) represented by [-A-B-O-]n·j M-Lj wherein A is a heterocycle such as a diazole, B is a ring compound like benzene, M is a metal of Groups 1 to 3 or Groups 12 and 13 in the periodic table, L is a substituent, and n is a valence of M. Examples cited for the organic metal complex (1) are iridium complexes such as Ir(ppy)3 and platinum complexes. The document states that a light-emitting layer comprising a conventional combination of an iridium complex such as Ir(ppy)3 and a host material such as CBP has problems of a decrease in the stability of the film structure as CBP tends to crystallize readily and of the driving stability and the document proposes to use the organic metal complex (2) as a host material. However, in spite of the fact that the number of the organic metal complexes (2) represented by the aforementioned formula is practically infinite because the substituent L can be chosen arbitrarily from an infinite variety of candidates, only the following two complexes are cited in the examples for concrete use of the organic metal complex (2): in one complex, A is a phenyl-substituted bezothiazole ring, B is a benzene ring, M is zinc, n is 2, and j is 0; in the other complex, A is a thiazole ring, B is a benzene ring, M is beryllium, n is 2, and j is 0.
The patent document 4 discloses a phosphorescent organic EL device wherein the light-emitting layer comprises a conventional iridium complex such as Ir(ppy)3 or platinum complex as a dopant and a compound containing an oxadiazole or triazole group as a host material and claims that such use of a compound containing an oxadiazole or triazole group makes it possible to form a light-emitting layer by a wet process and, as a result, the host material mixes sufficiently with the dopant and light of high brightness is emitted with minimal fluctuation. However, none of the oxadiazole or triazole compounds disclosed as host materials contains a metal and no description is given to teach that a host material is metal complex. The compound cited as a triazole or 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) has a property of facilitating the flow of electrons and obstructing the flow of holes and this shifts the light-emitting range to the side of the hole-transporting layer. Therefore, it is conceivable that the luminous efficiency from Ir(ppy)3 may drop depending upon the compatibility of Ir(ppy)3 with the material used for the hole-transporting layer. For example, 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) which is used most frequently in the hole-transporting layer on account of its excellent performance, high reliability, and long lifetime is poorly compatible with Ir(ppy)3 and the use of this compound raises a problem in that the transition of energy occurs from Ir(ppy)3 to NPB with the resultant drop in luminous efficiency.
The use of a material like 4,4′-bis(N,N′-(3-tolyl)amino)-3,3′-dimethylbiphenyl (HMTPD) to which the transition of energy from Ir(ppy)3 does not occur for the hole-transporting layer is conceivable as a means to solve the aforementioned problem, but this cannot be said to offer an excellent means from the viewpoint of luminous efficiency.